Your Airport Express is a modern marvel. I have one. I do not use it's built in DAC. Instead, I use the Airport Express SPDIF out to SPDIF input on my pre/pro, from which I can view bit rate. Thing is, whether you use the AE's DAC or not the AE up samples or down samples everything to 16/44.1. Some of my files, as large as 24/192, are being heard at 16/44.1. This means if you are not happy with 16/44.1 you would likely need to explore an alternative to AE. I am satisfied that 16/44.1 is all my ears can appreciate, so, the Airport Express is all I perceive I need for all that I was wanting-the best sound reproduction with the greatest convenience.
BTW, you do not need to add a DAC, you are getting digital to analog conversion from the Airport Express. It will process AAC, ALAC, and AIFF files no matter what the bit stream rate, again, up sampling or down sampling to 16/44.1. I guess you could add another DAC between AE and your RCA's on receiver to up sample using SPDIF from AE to SPDIF on DAC you have added but I do not think such up sampling will get you anything you could appreciate as sounding better; and, you would need a DAC you could manually set to up sample, that's going to be relatively expensive. You will have just added unnecessary complexity.